1. Field of the Invention
This invention relates to a device for detecting and adjusting the concentration of a developer for electrophotography or electrostatic recording which is a mixture of magnetic carrier and dielectric toner.
In electrophotography or electrostatic recording, a developer consisting of a carrier and a toner is usually used to develop electrostatic latent images. For example, in the magnet brush development, which image a magnet brush for the development and the conveyance of the developer at the same time time, the developer employed consists of a magnetic carrier such as iron powder or the like and a toner such as colored resin powder or the like, and one of the most essential factors which will affect the developing effect is the mixture ratio, i.e. the concentration, of the developer. More specifically, a portion of the developer which has been attracted to the magnet frictionally slides with respect to the electrostatic latent image so that the latent image is developed into a visible images by the toner in the developer, and during repetition of such process, the toner is gradually consumed from the developer to reduce its percentage with respect to the carrier and thus reduce the density of the developer, with a result that the density of the developed images is gradually reduced. To avoid this, toner must be suitably supplied but, if there is an oversupply of toner, the copy images will have too great a density as well as increased fog. Thus, the concentration of the developer must always be maintained at a proper level in order to continuously produce copy images of a desirable density. 2. Description of the Prior Art
A method of automatically controlling the concentration of the above-described developer is known. Where the developer used is a mixture of differently colored carrier and toner, the mixed color concentration of the developer is varied with the consumption of the toner. Utilizing this, the known method comprises optically detecting such variation to control a toner supply mechanism so as to effect toner supply to the developer in accordance with the detected variation, thereby maintaining the developer at a constant concentration. Also known is a method which utilizes the fact that the resistance value of the developer is varied with the consumption of the toner therein. This latter method comprises measuring said variation as a variation in a current inversely proportional to the resistance, to thereby detect the concentration of the developer, and adjusting the concentration of the developer in accordance with the detected signal.
The former method will not be available if the carrier and the toner are similar in color. Further, it is disadvantageous in that the color detecting optical system, including a light-emitting element, a light-sensing element, etc. is subject to contamination from scattered toner particles, which may result in failure to effect proper concentration detection. In the latter method, which comprises detecting the resistance variation to adjust the concentration, there is a problem that the very high resistivity of the toner in the developer permits only a very small current to flow through the developer, and as a result of which the variation in the current value for the variation in the resistance value is so small that it is very difficult to detect such current and adjust the concentration. This also imposes the necessity of providing a complicated amplifier, which in turn would lead to a higher error in the detection of the concentration and, accordingly, to malfunctioning of the device during toner supply.
Generally, the concentration adjustment has rarely taken into account the effect of the carrier deterioration or fatigue upon the detection of the carrier-toner ratio. The carrier deterioration is known as a phenomenon of the toner coming to cover the surface of the carrier in the developer as the development of electrostatic latent images is frequently repeated. As the carrier deterioration progresses, the color tone of the carrier approaches that of the toner to make the optical concentration detection method useless. Also, the resistance, particularly the dielectric breakdown voltage, of the developer is increased so much that some error will be involved in detecting the proper concentration value of the developer.
All these disadvantages may effectively be eliminated by the present invention.